Using a cohort of more than 300 subjects with a wide variety of eosinophilic disorders, ranging from benign eosinophilia to eosinophilic leukemia, we have continued to identify and characterize novel subgroups of patients with eosinophilia. Although most classifications of HES allude to a group of patients with absolute eosinophil count (AEC) >1500/mm3 in the absence of clinical manifestations (hypereosinophilia of unknown significance or HEUS), little is known about the characteristics and long-term prognosis of such patients in the absence of therapy. Among the 210 subjects >14 years of age with unexplained AEC >1500/mm3 evaluated at the NIH between January 1991 and December 2011, 36 were PDGFRA-negative and on no therapy at the time of evaluation. Of these, 8 (22%) were asymptomatic and remained eosinophilic (peak AEC 1856-7170/mm3) and without clinical manifestations of eosinophilia on no therapy for > 5 years (range 7-29 years). Peak eosinophil counts, surface expression of eosinophil activation markers, serum levels of IL-5, GM-CSF, IL-9, and IL-17A, and the prevalence of aberrant or clonal T cell populations were similar in the subjects with HEUS and untreated subjects with PDGFRAnegative HES (n=28). Interestingly, serum levels of IgE and IL-13 were significantly increased only in subjects with HES. Whereas these data suggest that completely asymptomatic patients with eosinophilia >1500/mm3 can be followed closely without specific treatment, a multicenter prospective study is planned to better assess the risk of and predisposing factors for progression to HES in patients with HEUS (Chen et al. 2013 J Allergy Clinical Immunology, in press). LHES is a rare eosinophilic disorder of unknown cause in which production of cytokines by clonal/and or aberrant T lymphocytes drives blood and tissue eosinophilia. We reported a case of LHES with dramatic dermatologic manifestations caused by an Epstein-Barr virus (EBV)-infected T cell clone producing eosinophilopoietic cytokines (IL-4, IL-5, and IL-13) (Klion et al. Blood 2013). EBV DNA was demonstrated in T cells in the peripheral blood (peak level 3.97 million copies/ml blood), skin and bone marrow using FISH and PCR, consistent with chronic active EBV infection (CAEBV). Elevated EBV DNA levels (11,800 copies/ml blood) were found in one additional patient with LHES and dermatologic disease (of 15 studied). Although these data suggest that CAEBV is not a common cause of LHES, EBV screening of patients with LHES and a demonstrable T cell clone should be performed to identify patients with CAEBV who may benefit from early consideration for hematopoietic stem cell transplantation. Other novel subgroups of eosinophilic subjects currently under study include a family with autosomal dominant eosinophilia and a cohort of subjects with cyclic eosinophilia. We have also initiated a new clinical protocol to determine whether the acute response to a single dose of prednisone can predict the dose ultimately needed to control eosinophilia and clinical manifestations of HES and to explore the mechanisms of glucocorticoid resistance in HES. The effects of novel therapies for the treatment of hypereosinophilic syndromes, including monoclonal antibodies to IL-5 and the tyrosine kinase inhibitor, imatinib, remain a primary focus. Long-term (5 year) followup of subjects enrolled on a placebo-controlled study of the anti-IL-5 antibody, mepolizumab was completed. The drug continued to show excellent efficacy in the majority of subjects and was not associated with toxicity or the development of neutralizing antibodies (Roufosse et al. 2012 J Allergy Clin Immunol). We have also begun to explore the therapeutic potential of antibodies against several additional novel targets, including EMR1 (Legrand J et al. Allergy Clin Immunol, in revision) and siglec-8 (Na et al. 2012 J Immunol Methods). Treatment of the filarial infection loiasis with diethylcarbamazine (DEC) can be associated with severe adverse events, including death, that are associated with a dramatic rise in eosinophilia and are positively correlated with the number of circulating microfilariae in the blood. In the past year, we have expanded our studies of monoclonal antibodies targeting IL-5 to explore the role of eosinophilia in post-treatment reactions following DEC treatment of loiasis. A double-blind, placebo-controlled study of the anti-IL-5 antibody, reslizumab, to prevent post-treatment side effects of DEC in patients with Loa loa infection is currently underway. Because similar side effects have been seen in Loa infected patients during mass administration of ivermectin for onchocerciasis control, we conducted a comparative study of post-treatment reactions following single dose ivermectin or single dose DEC. Of note, despite comparable side effect profiles, the patterns of post-treatment eosinophilia were dramatically different between the two groups. Analysis of cytokines and eosinophil activation markers is currently underway.